The present invention relates to a method for controlling a drive of a vehicle having at least a first axle with at least one mounted wheel, wherein a drive torque is set by a driver of the vehicle and the first axle is driven by a first electric machine as a function of the drive torque, and wherein a second electric machine which is coupled to at least one combustion engine supplies electrical energy for the purpose of driving the first axle by way of the first electric machine. The invention further relates to a vehicle with a drive and to a computer program product.
Vehicles of the generic type are known in principle, so that a specific documentation is not required. Vehicles, also known as motor vehicles, are used to travel on land. When in addition to an electric machine as a drive a combustion engine is additionally provided as a drive, these vehicles are usually referred to as hybrid vehicles.
In a vehicle with a first axle that can be driven exclusively by way of a first electric machine and with a hybrid axle, i.e. a second axle, which can be operated both by way of an internal combustion engine and by way of a second electric machine, the first electric machine can be supplied from a battery of the vehicle for driving the first axle. However, the electrical energy required for the operation of the first electric machine of the first, purely electrical driven axle may also be supplied by way of the internal combustion engine via the second electric machine operating in the generator mode. This operational state is referred to for the purposes of this disclosure also as compensation. A full compensation exists when the total electrical energy required for the operation of the first electric machine is supplied by the second electric machine. Arbitrary intermediate states may be assumed between the aforementioned states, for example, in that half of the total electrical energy required for the operation of the first electric machine is supplied by the battery of the vehicle and another half by the second electric machine. The first and the second electric machines are typically rotating electric machines, such as DC motors, three-phase motors or the like.
The electric machines are typically each driven by a respective inverter, which enables electrical energy to be converted between the respective electric machine and an intermediate circuit, to which the respective inverter is connected. Usually the intermediate circuit is formed by a DC-link circuit, to which at the same time the battery of the vehicle is also connected. The battery of the vehicle is typically designed as an accumulator.
If the battery of the vehicle is in an operating condition in which it can no longer receive or supply any significant amount of electrical energy any more, then a special operating status of the drive of the vehicle is required, in which exactly the amount of electrical energy is supplied by the second electric machine that is required for the operation of the first electric machine. To load the battery as little as possible, the generated and the consumed electrical energy must be identical as much as possible. Otherwise there is a danger of overloading the vehicle's battery. Such an operation of the drive is also referred to as serial operation. This operating condition can be realized by operating the second electric machine under current control. The current is then controlled with the inverter such that the generated current is exactly equal to the consumed current.
It has been proven to be disadvantageous in serial operation that is difficult to predict the torque produced by the second electric machine when operated in generator mode under closed-loop control. In addition, fluctuations may arise when additional electrical components, in particular electrical loads, are operated in the intermediate circuit of the vehicle.
In order to adjust a target torque on the hybrid axle, the internal combustion engine must provide exactly the target torque in addition to the torque required for operating the second electric machine. The internal combustion engine must hence always be adjusted to the changing torque. This is often not possible by using the same dynamics with which the changes take place. One consequence is that corresponding fluctuations become noticeable for the driver as changes in torque or changes in acceleration.
In addition, operating states between pure serial operation and normal operation are difficult to implement, because in normal operation the second electric machine is controlled as a generator and/or as a motor via a target torque, whereas in serial operation the second electric machine must be operated under current control. Significant problems arise when switching between the operating modes.